Field of the Invention
The invention relates to a method for the production of ultrasound waves for nondestructive materials testing, and to a test instrument.
Ultrasonic testing is a nondestructive materials testing method for detecting cracks, inclusions, non-uniformities and other defects. In this case, the ultrasound is, for example, produced piezoelectrically or electrodynamically.
With electrodynamic ultrasound production, the ultrasound is produced directly in the test piece, so that there is no need for a coupling medium. The creation of the ultrasound vibrations is attributable to the interaction of radio frequency eddy currents with a magnetic field. The eddy currents are, for example, produced by a radio frequency coil which is brought close to the surface of the workpiece. As a result of a magnetic field acting at the same time, Lorentz forces are created, which produce sound waves in the workpiece. Depending on the relative orientation of the magnetic field and the coil which produces the eddy currents, it is possible to excite longitudinal waves and arbitrarily polarized transverse waves. In the case of a longitudinal wave, the propagation direction and the vibration direction are identical, whereas with a transverse wave the vibration direction is perpendicular to the propagation direction. A transverse wave is also referred to as a shear wave and propagates only in solid media.
If the polarization direction lies in the plane spanned by the normal to a surface of the workpiece and the propagation direction of the ultrasound, then the waves are referred to as vertically polarized transverse waves. If, however, the polarization direction lies perpendicular to this plane, the waves are referred to as horizontally polarized transverse waves. For use in testing, horizontally polarized transverse waves can be produced only by electrodynamic excitation.
With electrodynamic ultrasound production, it is possible to test the workpiece at temperatures of up to about 1000 K.
German Patent DE 42 04 643 C1 discloses a testing instrument having permanent magnets which are oriented at right angles and whose orientation alternates in checker-board fashion. In this case, the orientation is defined as being the direction between the north and south poles of the permanent magnet. In this instrument, the conductive tracks of the radio frequency coil are arranged in meander fashion between a surface of the workpiece and the permanent magnets. This testing instrument is very expensive to produce, since the transmit or receive coil needs to be wound very thin in a planar shape. At the hitherto customary testing frequencies of about 0.7 MHz, this can be done only with great outlay. However, frequencies of between 1 and 2 MHz are usual for testing thin-walled components and pipelines. In order to achieve this, the permanent magnet arrangements and radio frequency coil arrangements need to be reduced in size according to the frequency. Reproducible production of testing instruments of this type therefore becomes very expensive.
European Patent EP 0 579 255 B1 discloses a further test instrument, in which the eddy currents needed for the excitation of sound are induced through the use of a magnet yoke enclosing the permanent magnets. The distance between the two pole pieces of the magnet yoke therefore becomes undesirably large. This testing instrument therefore has an efficiency, in terms of sound excitation and sound reception, which is strongly dependent on the material to be tested, and, by way of example, satisfactory results have not yet been achieved with nonmagnetic components. However, it is precisely with nonmagnetic weld seams and hybrid seams that, because of the columnar crystals through which sound is to be passed, it is particularly suitable to use horizontally polarized waves, which for all intents and purposes can only be produced electrodynamically.
A factor common to the test instruments known from the prior art, when using horizontally polarized transverse ultrasound waves, is that one or more radio frequency coils for exciting the ultrasound waves are arranged in magnetic fields which are arranged with alternating polarity and are produced by a multiplicity of permanent magnets. A problem which then arises is that, in order to change the angle of incidence .alpha., it is necessary not only to drive the individual radio frequency coils as a function of time, but also to change the excitation frequency of the ultrasound waves. In this case, in order to produce different angles of incidence .alpha., different excitation frequencies need to be used. An angle of incidence of 0.degree. cannot in this case be achieved. The angle of incidence .alpha. is defined as being the angle between the propagation direction of the ultrasound waves in the workpiece and the normal to the surface of the workpiece. The angle of incidence .alpha. has the following functional dependence ##EQU1## .lambda. being the wavelength of the ultrasound wave, and .lambda..sub.s being the track wavelength which is determined by the periodicity of the static magnetic field. Since the wavelength .lambda. depends on the frequency .nu., the angle of incidence .alpha. is therefore also dependent on the frequency .nu..